Optical Properties in Papermaking
Paper under light
For optical properties it is important that paper is porous (like snow).
Paper under light shows the following main phenomena and paper properties:
• Specular reflection gloss and smoothness
• Scattered reflection brightness and opacity
• Scattered absorption color and opacity
• Refraction opacity
• Transmission opacity
Snow - white
Ice – ”glassy”
Light reflection from printed image
For optical properties most important is how air is distributed in the paper and for strength
how fibers are distributed.
For good multicolor pictures it is important that paper reflects all wavelengths i.e. is white.
Picture: Evans, DuPont
Kubelka-Munk theory and light scattering
Paper is rough and porous material. This
means that it reflects scattered light from the
surface but also deeper from the paper.
Light scattering coefficient is a material
property, which tells how much from the
incoming light the material can reflect as
Light scattering coefficient can be calculated
from paper reflection measurements.
Refraction index of main paper components
are very similar. Every surface between
paper and air refracts light and scattering is
better when there are more these surfaces.
The higher the paper density is the less
there is light scattering.
Absorption coefficient can be calculated from the measured values of R∞ and
R0. It depends on the special chemical groups in molecules. Typically these
groups are present in all kind of dirt.
Lignin in a pulp has molecules which reflect only yellow light (=absorbs blue
light) and thus reduce brightness very much.
Pulp bleaching is basically additional removal of lignin from the pulp.
Reflection and transmission
R∞ is the reflection coefficient of so thick sheet pile that no light goes through. It
correlates with brightness measurements.
From the formula one can see that scattering coefficient must be high and absorption
coefficient low to get high brightness.
Several specialty papers require some or high transparency. These can be called
Reflection factor of a sheet against a black backing
Reflection factor of a large stack of sheets
ISO Opacity =
Reflection factor of a sheet against a black backing
Reflection factor of a sheet against a standard backing
Tappi Opacity =
100 % Opacity
Playing cards with black core
In the paper industry, there are two different
standards for opacity measurement. ISO
2471 (Printing Opacity) and TAPPI
T425 (Contrast Ratio) are the relevant
ISO 2471 utilizes the diffuse illumination
and 0° viewing geometry (d/0°) which is the
same as the ISO brightness geometry.
TAPPI T425 uses 15° illumination and diffuse
viewing(15°/d) originally developed by
Bausch & Lomb in the 1930's.
The illuminants of ISO and TAPPI are
different where effective wavelengths are 557
and 572 nm. Also the measured ISO and
TAPPI opacities are different.
ISO 2471 TAPPI T425
Geometry d/0° 15°/d
White Backing R∞ R0.89
Wavelength, nm 557 572
Opacity and brightness
Opacity % is 100*R0/R∞. This simple formula
tells that opacity is lower when brightness is
It is very difficult to get high opacity when
brightness is high. This is the reason that
newsprint grammage can be about 45 gsm but
copy paper must be about 80 gsm.
Opacity relates to the show-through of the
printed image from the opposite side of the
sheet, or the sheet under it.
A = 2856 ºK, “electric lamp”
B = 4874 ºK
C = 6504 ºK, ”indoor daylight”
D65 = 6774 ºK, ”outdoor daylight”D65 = more UV
D65 is outdoor daylight, where UV is fully included. C is about indoor daylight (less UV).
Illuminants C and D65 are used for paper measurements. The big difference is that D65
includes more UV light (wavelengths less than 400 nm).
Optical brighteners (OBA = FWA) convert this UV light to visible blue light thus
increasing brightness values.
There are two basic types of brightness
measurements: directional and diffuse.
Directional brightness (TAPPI brightness – TAPPI
452) employs the 45º/0º geometry of the original
GE-Photovolt instrument. It has been the standard
in the U.S. and Japan. Fiber orientation has effect
on the TAPPI brightness measurement.
Diffuse brightness employs a D/0º geometry where
D indicates diffuse illumination from a sphere,
making it insensitive to sample orientation. Diffuse
brightness is the standard in much of the rest of
Two types of diffuse brightness are commonly
discussed, and a standard exists for both of them.
ISO C Brightness (ISO 2470-1, TAPPI 525) uses
Illuminant C level of UV energy. It simulates
normal office lighting conditions.
ISO D65 Brightness uses Illuminant D65 (daylight)
according to ISO 2470-2.
D65 illuminant and measuring slot of brightness
For brightness measurement a narrow wavelength ”slot” has been standardised. The
dominant wavelength of this slot is 457 nm (blue) and the range is ± 44 nm. If paper is
yellow it reflects less blue light. This measure is effective for bleaching (less lignin less
D65 illuminant includes lot of UV light, which can be converted to blue light by using optical
brighteners. This increases brightness.
When measuring and calculating whiteness all wavelength have effect and blue dyes
350 400 450 500 550 600 650 700
Brightness is measured from the slot
which is under the white curve.
Brightness weighting function
When pulp is bleached the reflectance of all wavelengths increases. However, the
increase is largest at blue end of the spectrum.
Pulp brightness measurement is not only the measurement of total reflection but
especially the increase of blue reflection (or decrease of yellow lignin absorption).
UV-light and copy paper reflection spectrum
Brightness is measured with dominant wavelength of 457 nm. Illuminants C and
especially D65 give higher brightness when optical brighteners are used. Actually
reflection is lower close to 400 nm, where absorption is higher and the energy is moved to
Picture: Nils Pauler
Whiteness is the ratio of Red, Green and Blue reflectance. It is an attribute of a
diffusing surface which denotes its similarity in color to preferred or standard white.
Measurable properties CIE Whiteness (ISO 11475) and tint equations can be stated as
• W = 2.41L* – 4.45b*(1–0.009(L*–96))–141.4
• T = –1.58a*–0.38b*
Brightness and whiteness of copy papers
D65 brightness of European copy papers can be more than 100% with D65 light and
optical brightening agents.
Whiteness is about 50 %-unit higher than conventional ISO-brightness and D65
brightness about 10 %-unit higher than ISO brightness with C illuminant.
1 2 3 4 5 6 7 8 9 10 11
Brightness, C/2º, ISO 2470:1999
Brightness, D65/10º, SCAN P-66
Whiteness, D65/10º, ISO 11475:1999
Fluorescent whitening agent (FWA or OBA)
Fluorescent Whitening Agent (FWA or OBA) is used to increase the white appearance of
papers by absorbing invisible ultraviolet light and re-emitting it in the blue region of the
visible spectrum. It is widely used in Europe to make bright surface.
This strategy can compensate for a yellow tint of many types of pulps that have been
bleached to moderate levels.
Fluorescent component of brightness
Fluorescent component is the additional brightness obtained from the use of optical
brighteners. It is determined by measuring the sample with and then without the effect of
UV energy on the sample.
When comparing brightness it is important to know what method and light has been
used. The following data is an example of differences.
D65 ISO TAPPI CIE D65 CIE C ISO-TAPPI D65 -ISO
A 98,9 91,8 89,9 126,6 106,0 1,9 7,1
B 95,3 89,0 87,3 117,5 98,6 1,7 6,4
C 105,6 95,1 91,8 139,3 112,3 3,3 10,5
D 111,2 99,8 96,5 161,9 134,5 3,3 11,4
E 110,6 98,9 95,5 162,0 133,9 3,4 11,7
F 112,3 100,4 96,0 149,7 122,2 4,4 11,8
G 113,5 100,9 96,4 160,5 132,2 4,5 12,6
H 110,1 98,7 94,7 145,3 117,4 4,0 11,4
I 112,3 100,2 95,9 161,2 133,6 4,3 12,1
J 93,0 88,6 90,0 110,8 98,9 -1,4 4,4
K 102,0 92,5 90,6 130,9 106,5 1,9 9,5
L 110,1 99,1 95,0 152,5 126,3 4,1 11,1
M 110,1 99,5 96,1 150,6 124,5 3,4 10,7
N 107,2 96,7 94,1 153,6 127,9 2,6 10,6
O 106,9 96,3 93,0 143,0 115,7 3,3 10,6
P 110,3 99,2 96,1 146,3 119,0 3,1 11,1
Q 109,6 99,0 96,1 143,5 118,1 2,9 10,5
R 113,9 101,5 97,4 149,9 121,8 4,1 12,4
S 111,6 99,3 95,0 163,0 134,6 4,3 12,3
Brightness Elrepho Whiteness Elrepho
Paper gloss metering principle
Tappi T 480 defines the specular gloss of paper and paperboard at 75 degrees (15 °
from the plane of paper). This method is suitable for low to moderate-gloss coated and
uncoated papers as well as for most ink films on paper or paperboard.
The standard describes the technical requirements for a corresponding gloss meter.
Gloss measurements are normally made using standard equipment like Hunter,
Lehman and Zehntner and giving a single mean gloss value. But, a sample can have a
high mean gloss value (normally considered as good) but at the same time have a high
gloss variation which is disturbing when looking at a printed picture. The micro gloss
method can quantify such disturbing gloss textures and ‘glare effects’ on printed and
Example of gloss instrument
It must be remembered that paper gloss is
different in different directions depending on
fiber orientation. Machine direction gloss is
highest and cross machine gloss lowest.
The Technidyne PROFILE/Plus Gloss
automatically measures the gloss at 75º in the
MD and CD according to the following Industry
Standards: TAPPI Method T 480, ISO 8254-1.
Uncoated paper raw materials
Mechanical printings and woodree uncoated papers.
Material Mech. % WF % Comment
Fibers 60 – 100 70 - 100 Wood or non-wood fibres
Fillers 40 – 0 30 - 0 Mineral or synthetic pigments
Surface size - 0 - 5
Starch, CMC, PVA, synthetic size,
optical brighteners etc.
0 – 1 0 – 2
Internal sizes, dyes etc.
(effect on paper properties)
Retention aids, defoamers, biocides etc.
(effect on process performance)
Water 5 – 10 4 – 7 To be in balance with air humidity
Fillers and coatings in papers
To improve optical properties of paper mineral pigments are used in
papermaking. They can be added as a filler before headbox or to the surface
as a coating with binders.
0 - 5
5 - 15
0 - 1.5
Unctd Mechanical, TD, Bulky
5 - 15
15 - 35
0 - 5
Ctd Mechanical, LWC
5 - 15
8 - 18
0 - 2
5 - 15
20 - 40
Uncoated Woodfree, Copy
15 - 30
10 - 25
1 - 2
1 - 2
0 - 5
Coated Woodfree, standard
10 - 15
12 - 18
0 - 2
0 - 2
10 - 15
20 - 35
Light scattering coefficient of pulps
Finer fibers and less bonding give better
light scattering (opacity & brightness)
When mechanical pulps have lower
freeness light scattering is better (more
refining, more unbonded fines)
When chemical pulps have lower
freeness light scattering is lower (more
bonding, practically no unbonded fines)
Harwood has better scattering than
The more chemicals in pulping is used
the lower light scattering will be
Brightness and chromophores
Brightness is not increasing linearly in bleaching. To make very bright pulp requires too
much effort and bleaching chemicals.
It is good to know in papermaking that very small amounts of lower brightness
components, such as mechanical pulp, low brightness clay or dirty process water
reduces brightness very fast.
Main pigment requirements
Effects of filler addition
Drying shrinkage dimensional
Brightness, opacity and color
Ink absorption more uniform
Smoothness and gloss
Costs and printability
All general strength properties
Surface strength , dusting
Internal bond strength
Carbonate requires > 7 pH
Retention , two-sidedness
Wire, felt and machine wear
With PCC bulk and porosity can increase, with other fillers they decrease
Fillers - agglomerated or dispersed?
Fillers should be first fixed to the fibres when they are dispersed. This
would guarantee good retention, strength and optical effect (brightness
and opacity) at the same time.
Fillers well dispersed
Filler can either fill paper pores (left) or distribute evenly on fibre surfaces
(right). Even distribution has good optical effect but reduces strength more.
Good filler distributionBad filler distribution
Pictures: Robert A Gill
Main raw material requirements
High light scattering improves opacity and brightness. High absorption coefficient
improves opacity but decreases brightness.
Fillers and pigments are good raw materials for optical properties (low k and high s).
Property s k
High brightness + –
High opacity + +
Total mineral content of paper & board
Main fillers and coating pigments
Titanium dioxide is a special filler with high refractive index. It is the only possibility
to improve opacity of impregnated and waxed papers. This is the reason that décor
paper includes titanium dioxide.
When making opacity and brightness TiO2 is a very expensive filler or coating
pigment. It is used in U.S. for this purpose but not in Europe where brightness and
opacity are made with less expensive means.
Titanium dioxide price is more than three times pulp price while carbonate filler price
can be less than half of pulp price.
Mineral Avg size Refractive Scattering Brightness Density Hardness
Pigment Index coefficient
/g % kg/m
Clay 0,2-2,0 1,55-1,57 1100-1200 80-92 2580 2-2,5
Calcined clay 0,7-1,5 1,60 2600-3000 90-95 2600 4,0-5,0
GCC 0,7-3,0 1,5-1,7 1400-1700 85-95 2710-2930 3,0-4,0
PCC 0,3-3,0 1,5-1,7 2200-6000 96-100 2710-3830 3,0
TiO2 Anatase 0,2-0,4 2,5-2,55 4500-6000 98-100 3820-3970 5,5-6,0
TiO2 Rutile 0,2-0,4 2,6-2,9 4500-6000 98-100 4230-5500 6,0-7,0
Effect of filler content on tensile strength
Dry tensile strength is reduced about 50% when a normal 20% loading is
used. Initial wet strength reduces even more.
Higher particle size gives better strength but optical effect will be lower
due to lower light scattering.
Picture: Robert A Gill
Filler content, %
Particle size increases
Critical properties of titanium dioxide
Anatase, RI = 2.5
Rutile, RI = 2.7
Several properties of TiO2 are different compared to other fillers. However, if the effects
are compared at the same level of opacity increase, the detrimental effects with TiO2
are lower than with several other fillers and pigments.
Even if the price of TiO2 can be up to ten times compared to lowest price fillers, the
cost can be lower because the usage can be only 10% of the use of main filler (1-2% of
paper for printing papers).
Because the share of TiO2 as a filler is low a good retention is very important.
TiO2 absorbs UV light and the effect of OBA is reduced with TiO2. OBA is more used in
Europe and TiO2 in North America.
Minerals in papermaking
Minerals are a fast growing raw materials of papermaking. Total amount of minerals in
paper and board is globally over 10%. We are back in stone age.
Especially consumption of carbonates has been growing fast because they are white,
easy to find everywhere and less expensive than fibers or clay.
Stone Forest in Kunming, China
Paper Color and Printing
Illuminant Eye and brain
When lights are mixed it is additive mixing (we add energy). When inks and paints are
mixed it is subtractive mixing (we add absorption and reduce energy from reflection).
Pure red and green light produce yellow, red and blue make magenta, blue and green
combine to make cyan, and all three together, when mixed at full intensity, create white.
For mixing of dye pigments, it is better to use the secondary colors, since they mix
subtractively instead of additively. Using Cyan, Yellow and Magenta toners we can create
colors on paper.
In digital printing, when we talk about “gamut”
we mean the color space of a device.
Devices can be divided into two categories:
• RGB devices like scanners, monitors, digital
• CMYK devices like laser printers, offset etc.
Normally, a CMYK printer device color space
will be smaller (less saturated and fewer colors)
than a RGB capture device color space.
A typical RGB color space
A typical CMYK color space
RGB color system
Light with a wavelength between 600 and 700 nm is known as red light.
Light with a wavelength between 500 and 600 nm is known as green light.
Light with a wavelength between 400 and 500 nm is known as blue light.
By combining Red, Green and Blue light we can create all the colors of the
400 500 600 700 nm
Blue Green Red
Blue Green Red
Cyan 1 1 0
Magenta 1 0 1
Yellow 0 1 1
1 = Reflection
0 = Absorption
White paper reflection
In theory, white paper reflects all colors.
This is a theoretical statement, because different brands of paper have a different color.
This is why in color management it is very important to know what paper we are using.
This is not only true for the output, but also for the original.
Yellow toner absorbs blue light
Yellow is the complement of blue.
Yellow toner absorbs blue light and reflects green and red light.
The reflected “G” and “R” light are seen as yellow.
Magenta toner absorbs green light
Magenta is the complement of green.
Magenta toner absorbs green light and reflects blue and red light.
The reflected “B” and “R” light are seen as magenta.
Cyan toner absorbs red light
Cyan is the complement of red.
Cyan toner absorbs red light and reflects green and blue light.
The reflected “B” and “G” light are seen as cyan.
Mixing subtractive colors
Equal amounts of magenta and yellow toner produces red.
Equal amounts of cyan and yellow toner produces green.
Equal amounts of magenta and cyan toner produces blue.
In theory, equal amounts of C, M and Y produce black.
This black is called “Process Black”.
In reality, it is virtually impossible to produce true black using cyan, magenta and
yellow toner. Depending on the used toners or inks, the result can vary form
deep blue to be brown or gray.
Pure black Process black
Yellow school bus
Yellow surface absorbs mainly blue rays.
The CIE Lab model
CIE Lab is the second of two
systems adopted by the CIE. It is an
attempt to reduce the distortion in
Lab is based on XYZ, but is non-
linear, to try to mimic the human
• L is a luminance scale.
• a and b are color axes.
Although not perfect, it is the most
useful system today.
Lab colour system
Vertical axis L is black and
b is yellow-blue axis. Bright
paper has negative b values
i.e. paper is bluish.
a is red-green axis. Bright
paper has positive a values i.e.
paper is reddish.
Blue-red paper looks brighter
Multicolor process CMYK
Papermaking variables and paper properties
Source: Michael Evans
Colour of white papers
Kuva: Jouni Marttila
Close to the neutral point all papers look white. When looking more closely,
higher quality papers are bluish (-b values) and lower quality papers are
yellowish (+b values).
Example of commercial papers
Brightness ISO (%) in relation to Opacity
Development of NorCal in relation to the competition > Measurements resulted from print trials.
Stora Enso Publipress matt
Stora Enso Envi Press
SCA Grapho verde
Stora Enso maxau
SCA Grapho verde
Brightness C2 (%)
MY Joy Blue
AF&PA brightness comparison
81.9 and below
GE BRIGHTNESS METER
How to improve brightness and opacity?
Action s k brightness opacity
Bleaching –– ++ –
Mechanical pulp refining + +
Chemical pulp refining –– – –
Adding carbonate filler ++ – + ++
Adding OBA/FWA – +
Adding blue dye + – +
Adding other dyes ++ –– ++
Wet pressing – – –
Drying paper more + + +
Surface sizing – – –
Calendering – – –